F1 + F2. There you have your screenshot, can be found in the .minecraft directory in the folder screenshots.
Also if anyone didn't know, F3 toggles an statistics overlay, shows which coordinates you're at, how much RAM is used, how many chunks are loaded, how many FPS you have and a graph that shows lagspikes.
Then there's F5, toggles third person view and F8, good for fimling because it mimics a steady cam.